# std::unique_lock<Mutex>::unique_lock

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< cpp‎ | thread‎ | unique lock

unique_lock() noexcept; | (1) | (since C++11)
unique_lock( unique_lock&& other ) noexcept; | (2) | (since C++11)
explicit unique_lock( mutex_type& m ); | (3) | (since C++11)
unique_lock( mutex_type& m, std::defer_lock_t t ) noexcept; | (4) | (since C++11)
unique_lock( mutex_type& m, std::try_to_lock_t t ); | (5) | (since C++11)
unique_lock( mutex_type& m, std::adopt_lock_t t ); | (6) | (since C++11)
template< class Rep, class Period > unique_lock( mutex_type& m, const std::chrono::duration<Rep, Period>& timeout_duration ); | (7) | (since C++11)
template< class Clock, class Duration > unique_lock( mutex_type& m, const std::chrono::time_point<Clock, Duration>& timeout_time ); | (8) | (since C++11)

Constructs a `unique_lock`, optionally locking the supplied mutex.

1) Constructs a `unique_lock` with no associated mutex.
2) Move constructor. Initializes the `unique_lock` with the contents of other. Leaves other with no associated mutex.
3-8) Constructs a `unique_lock` with m as the associated mutex. Additionally:
3) Locks the associated mutex by calling m.lock().
4) Does not lock the associated mutex.
5) Tries to lock the associated mutex without blocking by calling m.try_lock(). The behavior is undefined if `Mutex` does not satisfy Lockable.
6) Assumes the calling thread already holds a non-shared lock (i.e., a lock acquired by `lock`, `try_lock`, `try_lock_for`, or `try_lock_until`) on m. The behavior is undefined if not so.
7) Tries to lock the associated mutex by calling m.try_lock_for(timeout_duration). Blocks until specified timeout_duration has elapsed or the lock is acquired, whichever comes first. May block for longer than timeout_duration. The behavior is undefined if `Mutex` does not satisfy TimedLockable.
8) Tries to lock the associated mutex by calling m.try_lock_until(timeout_time). Blocks until specified timeout_time has been reached or the lock is acquired, whichever comes first. May block for longer than until timeout_time has been reached. The behavior is undefined if `Mutex` does not satisfy TimedLockable.

### Parameters

other | - | another `unique_lock` to initialize the state with
m | - | mutex to associate with the lock and optionally acquire ownership of
t | - | tag parameter used to select constructors with different locking strategies
timeout_duration | - | maximum duration to block for
timeout_time | - | maximum time point to block until

### Example

Run this code

    #include <iostream>
    #include <mutex>
    #include <thread>
    #include <utility>
    #include <vector>

    std::mutex m_a, m_b, m_c;
    int a, b, c = 1;

    void update()
    {
        {   // Note: std::lock_guard or atomic<int> can be used instead
            std::unique_lock<std::mutex> lk(m_a);
            ++a;
        }

        {   // Note: see std::lock and std::scoped_lock for details and alternatives
            std::unique_lock<std::mutex> lk_b(m_b, std::defer_lock);
            std::unique_lock<std::mutex> lk_c(m_c, std::defer_lock);
            std::lock(lk_b, lk_c);
            b = std::exchange(c, b + c);
        }
    }

    int main()
    {
        std::vector<std::thread> threads;
        for (unsigned i = 0; i < 12; ++i)
            threads.emplace_back(update);

        for (auto& i : threads)
            i.join();

        std::cout << a << "'th and " << a + 1 << "'th Fibonacci numbers: "
                  << b << " and " << c << '\n';
    }

Output:

    12'th and 13'th Fibonacci numbers: 144 and 233

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